Any light signal can be expressed in the form of a spectrum S(λ), where λ denotes a wavelength, the significant values of which are generally between 380 and 780 nm (nanometers) in the visible region. By means of digital processing, this spectrum is projected onto a colorimetric coordinate system specific to a particular colorimetric system. The best known of these colorimetric systems is the trichromatic Red, Green, Blue system (hereinafter known as the RGB system). Thus a triplet of colorimetric coordinates is obtained corresponding to the expression of the light signal in said particular colorimetric system. These coordinates correspond to light intensity values respectively associated with three chromatic components of the light signal, called R, G and B in the RGB system. Each one is encoded over N data bits, such that a triplet is encoded over 3N data bits. The invention is described hereinafter in its application to a digital light restoration unit operating according to the RGB system. However, this choice is not limiting, and the invention may also be applied, in particular, to a digital light restoration unit operating according to other conventional colorimetric systems, such as the XYZ system.
Video projectors which operate according to the RGB system (hereinafter called RGB video projectors) accept digital input data defining, in the RGB system, the colorimetric coordinates of the light signal to be restored. For an image of particular dimensions projected onto a white screen, they can produce brightness values belonging to a set of discrete values. It may be recalled that the brightness is a quantity which characterizes the intensity of the light perceived by the human visual system. The brightness is therefore related to the light intensity emitted by the video projector, in relation to the surface area of the image projected onto the screen. It is measured in cd/m2 (candela per square meter). In the present account, it is considered that a video projector generates an image of particular dimensions onto a screen, for example an image of 1 m2, such that brightness values rather than light intensity values are discussed. In the case of a colorimetric system such as the RGB system, the brightness is a nonchromatic additive quantity. Consequently, the brightness at any point of an image generated from an RGB video projector is substantially equal to the sum of the brightnesses associated with each R, G and B component at this point.
For RGB video projectors which are currently available, the digital input data are encoded over 24 bits, 8 bits for each R, G and B component. In other words, for these video projectors, the value of N is eight (N=8). It is thus possible to obtain up to 224, that is nearly 16 500 000 different colors or hues. Each colorimetric coordinate therefore corresponds, for the corresponding colorimetric R, G or B component, to a brightness value included within a set of 28=256 possible discrete values. These values, respectively called LR, LG and LB, are denoted by a level between a minimum level 0 (zero brightness) and a maximum level 255 (maximum brightness). The maximum brightness value of any light signal is theoretically obtained by setting the three R, G and B components to the level 255.
Currently, RGB video projectors are available covering a dynamic brightness range from 0 to about 250 cd/m2. Assuming that these R, G and B components are equal, (hues are discussed in terms of gray levels to denote the 28=256 possible hues), such a video projector then has brightness steps substantially equal to 1 cd/m2. Such brightness steps are small enough to enable color transitions to be produced which, for the human visual system, appear overall to be continuous. However, it is currently sought to increase the dynamic brightness range of the video projectors in order to obtain images which are closer to reality. It would of course be possible to manufacture video projectors covering a higher dynamic brightness range, for example from 0 to about 250 000 cd/m2, by providing them with a more powerful light source. On the other hand, this higher dynamic range would be covered by correlatively higher brightness steps, substantially equal to 100 cd/m2, which would no longer make it possible to obtain color transitions perceived overall as continuous. The result of this is that, for example, the display of a color degradation would appear to the user as a succession of bands of discontinuous color in the optical spectrum. This drawback is associated with the technology of video cards which are currently used for and in the video projectors, and which process digital input data encoded over at the most 8 bits. Now, at present there are no video cards available which process digital data encoded over a larger number of bits, which would make it possible to avoid this problem.
One aim of the invention is to enable a light signal to be restored with a dynamic brightness range which is higher than that of the video projectors currently available, without incurring the aforementioned drawback.